JP2018042073A - Optical transmitter, optical transmission system and optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve signal quality deterioration.SOLUTION: An optical transmitter includes: a transmission signal generation unit which generates a plurality of low-speed signals from a transmission data sequence; a high-speed signal generation unit which divides the plurality of low-speed signals into each combination which includes two or more low-speed signals, and performs the digital-to-analog conversion of the plurality of low-speed signals to synthesize for each combination to generate a high-speed signal; a transfer function estimation unit which calculates a transfer function on the basis of the generated high-speed signal and a reference signal; and a transfer function compensation unit which compensates the plurality of low-speed signals generated by the transmission signal generation unit according to the transfer function to output to the high-speed signal generation unit.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmitter, an optical transmission system, and an optical receiver.

  In the backbone network of an optical communication system, due to the expansion of communication traffic in recent years and the increase in the capacity of lines for accommodating client signals such as 400 Gbps (Gigabits per second) and 1 Tbps (Terabits per second) Ethernet (registered trademark), There is a demand for an increase in transmission capacity per channel. In a current system having 100 Gbps per channel, a modulation rate is 32 GBaud (Baud), and a digital coherent optical transmission method using DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) as a modulation method is adopted (for example, Non-Patent Document 1).

  In order to expand the transmission capacity per channel, in the next-generation 400 Gbps and 1 Tbps class transmission, improvement in modulation speed is being studied (for example, see Non-Patent Document 2). Limiting the analog band of DAC (Digital to Analog Converter) is an issue in improving the modulation speed. As a technique for expanding the analog band of the DAC, a technique for synthesizing a plurality of low-speed signals in an analog manner to generate a high-speed signal has been proposed (see, for example, Non-Patent Documents 3 and 4).

Joe Berthold et.al, “100G Ultra Long Haul DWDM Framework Document”, OIF (OPTICAL INTERNETWORKING FORUM), June 30, 2009. G. Raybon, AL Adamiecki, P. Winzer, C. Xie, A. Konczykowska, F. Jorge, J.-Y. Dupuy, LL Buhl, S. Chandrashekhar, S. Draving, M. Grove, and K. Rush, "Single-carrier 400G interface and 10-channel WDM transmission over 4,800 km using all-ETDM 107-Gbaud PDM-QPSK" in Optical Fiber Communication Conference / National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America , 2013), paper PDP5A.5. H. Yamazaki et al., “160-Gbps Nyquist PAM4 Transmitter Using a Digital-Preprocessed Analog-Multiplexed DAC” Optical Communication (ECOC), 2015 European Conference on, Valencia, 2015, pp. 1-3. X. Chen, S. Chandrasekhar, S. Randel, G. Raybon, A. Adamiecki, P. Pupalaikis, and P. Winzer, "All-electronic 100-GHz Bandwidth Digital-to-Analog Converter Generating PAM Signals up to 190- GBaud "in Optical Fiber Communication Conference Postdeadline Papers, OSA Technical Digest (online) (Optical Society of America, 2016), paper Th5C.5.

  However, in the techniques described in Non-Patent Documents 3 and 4, a plurality of DACs are used when generating a high-speed signal based on a low-speed signal, and an analog device is used for the synthesis unit. For this reason, there is a problem that signal quality is deteriorated due to a difference in frequency characteristics between low-speed signals due to individual differences of DACs and imperfections of analog devices.

  In view of the above circumstances, an object of the present invention is to provide an optical transmitter, an optical transmission system, and an optical receiver that can improve the degradation of signal quality.

  According to one aspect of the present invention, a transmission signal generation unit that generates a plurality of low-speed signals from a transmission data sequence, and the plurality of low-speed signals are divided into combinations including two or more low-speed signals, and the plurality of low-speed signals are A high-speed signal generation unit that generates a high-speed signal by performing digital-to-analog conversion and combining each combination, a transfer function estimation unit that calculates a transfer function based on the generated high-speed signal and a reference signal, and And a transfer function compensation unit that compensates the plurality of low-speed signals generated by the transmission signal generation unit based on the transfer function and outputs the compensated signal to the high-speed signal generation unit.

  One aspect of the present invention is the optical transmitter described above, wherein the transfer function estimation unit calculates the transfer function using the plurality of low-speed signals generated by the transmission signal generation unit as the reference signal, or The transfer function is calculated by using a part of the transmission data sequence determined in advance as a known signal as the reference signal, or the low-speed signal is generated from the symbol sequence obtained by determining the received symbol of the high-speed signal Then, the transfer function is calculated using the generated low-speed signal as the reference signal.

  One aspect of the present invention is an optical transmission system including an optical transmitter, an optical receiver, and a transmission path connected to the optical transmitter and the optical receiver, and the optical transmitter includes: A transmission signal generation unit that generates a plurality of low-speed signals from a transmission data sequence, and the plurality of low-speed signals are divided into combinations including two or more low-speed signals, and the plurality of low-speed signals are digital-to-analog converted to the combination A high-speed signal generation unit that generates a high-speed signal by combining the optical signals, and an optical modulator that transmits an optical signal obtained by modulating the high-speed signal to the transmission path, and the optical receiver includes the optical receiver, A receiving unit that receives the optical signal from a transmission path and outputs the high-speed signal obtained from the received optical signal, and binary information included in the transmission data series based on the high-speed signal output from the receiving unit Demodulate A high-speed signal that is provided in the optical transmitter or the optical receiver and is generated by the high-speed signal generator, or the high-speed signal that is generated in the optical receiver. A transfer function estimating unit that calculates a transfer function based on any one of the high-speed signal and a reference signal, the optical transmitter, or the optical receiver, and the optical transmitter or the optical receiver. An optical transmission system comprising: a transfer function compensation unit that compensates the plurality of low-speed signals generated in a receiver based on the transfer function.

  One aspect of the present invention is the above-described optical transmission system, wherein the transfer function estimation unit calculates the transfer function using the plurality of low-speed signals generated by the transmission signal generation unit as the reference signal, or The transfer function is calculated by using a part of the transmission data sequence determined in advance as a known signal as the reference signal, or the received symbol of the high-speed signal is determined, and the low-speed signal is obtained from the symbol sequence obtained by the determination. A signal is generated, and the transfer function is calculated using the generated low-speed signal as the reference signal.

  One aspect of the present invention is the above-described optical transmission system, wherein the transfer function estimating unit performs the waveform shaping when the transmission signal generating unit performs waveform shaping to compensate for waveform distortion in the transmission path. The plurality of low-speed signals that are not performed are used as the reference signal, or the transfer function is calculated after performing the same waveform shaping on the high-speed signal.

  In one embodiment of the present invention, a plurality of low-speed signals are divided into combinations including two or more low-speed signals, and the plurality of low-speed signals are digital-analog converted and synthesized for each combination to generate a high-speed signal. An optical receiver that receives an optical signal transmitted by an optical transmitter, based on a receiving unit that receives the optical signal, the high-speed signal obtained from the optical signal received by the receiving unit, and a reference signal A transfer function estimating unit that calculates a transfer function, a transfer function compensating unit that compensates the plurality of low-speed signals obtained from the optical signal received by the receiving unit based on the transfer function, and the transfer function compensating unit A signal synthesizer for synthesizing the plurality of low-speed signals to be compensated to generate the high-speed signal, and reception for demodulating binary information included in the transmission data sequence based on the generated high-speed signal And over data demodulator, an optical receiver comprising a.

  One aspect of the present invention is the optical receiver described above, wherein the transfer function estimation unit receives the plurality of low-speed signals from the optical transmitter, and uses the received plurality of low-speed signals as the reference signal. The transfer function is calculated, or the transfer function is calculated by using a part of the transmission data sequence defined in advance as a known signal as the reference signal, or the high-speed signal obtained from the received optical signal Received symbols are generated, the low-speed signal is generated from the symbol sequence obtained by the determination, and the transfer function is calculated using the generated low-speed signal as the reference signal.

  One aspect of the present invention is the above-described optical receiver including a high-speed signal separation unit that separates the high-speed signal obtained from the optical signal received by the reception unit and generates the plurality of low-speed signals. The transfer function compensation unit compensates the plurality of low-speed signals generated by the high-speed signal separation unit based on the transfer function.

  According to the present invention, it is possible to improve degradation of signal quality.

It is a block diagram which shows the structure of the optical transmitter in 1st Embodiment. It is a block diagram which shows the structure of a transmission signal generation part. It is a block diagram which shows the structure of a high-speed signal generation part. It is a block diagram which shows the structure of a transfer function estimation part. It is a block diagram which shows the other structural example of an optical transmitter. It is a block diagram which shows the structure of the optical transmission system in 2nd Embodiment. It is a block diagram which shows the structure of the optical transmission system in 3rd Embodiment. It is a block diagram (the 1) which shows the structure of the transfer function estimation part applied to an optical receiver. It is a block diagram (the 2) which shows the structure of the transfer function estimation part applied to an optical receiver. It is a graph which shows the experimental result using the optical transmission system of 2nd Embodiment.

(First embodiment)
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration of an optical transmitter according to the first embodiment. The optical transmitter 10 includes a transmission signal generation unit 11, a transfer function compensation unit 12, high-speed signal generation units 13-1 to 13-4, an optical modulator 14, and a transfer function estimation unit 15. In the following description, the meanings of low speed and high speed in the terms of low speed signal and high speed signal indicate the narrowness and breadth of the bandwidth of the analog band. For example, the bandwidth of the high speed signal is fc [GHz]. In some cases, the bandwidth of the low-speed signal is smaller than fc [GHz]. In this embodiment, the bandwidth is about high-speed signal≈low-speed signal × N (N is an integer and the number of DACs).

  The transmission signal generation unit 11 has the internal configuration shown in FIG. 2 and includes a bit mapping unit 111, a waveform shaping unit 112, and a high-speed signal generation unit pre-signal processing unit 113. The bit mapping unit 111 assigns transmission bits to symbol points such as QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) with respect to a transmission data sequence that is binary information supplied from the outside, and provides a high-speed signal. A modulated signal sequence (XI, XQ, YI, YQ) is generated. The waveform shaping unit 112 performs a filtering process such as a Raised Cosine Filter or a Root Raised Cosine Filter on the modulated signal sequence output from the bit mapping unit 111. The waveform shaping unit 112 may perform arbitrary signal processing such as pre-chromatic dispersion compensation or pre-linear optical effect compensation for compensating for waveform distortion in the transmission path.

  The high-speed signal generation unit pre-signal processing unit 113 performs signal processing so that a desired signal is generated when the low-speed signals to be output are combined by the high-speed signal generation units 13-1 to 13-4. Generate a slow signal. As a technique for generating a low-speed signal, for example, there is a technique shown in Non-Patent Document 3. In the method shown in Non-Patent Document 3, the modulated signal sequence (XI, XQ, YI, YQ), which is a high-speed signal filtered by the waveform shaping unit 112, is divided in the frequency domain, the high-frequency component is folded, and the low-frequency component and The added signal and the signal subtracted from the low frequency component are each output as a low speed signal. Further, the high-speed signal generation unit pre-signal processing unit 113 may generate a low-speed signal using any method other than the method shown in Non-Patent Document 3.

  The transfer function compensation unit 12 performs frequency characteristic compensation in the high-speed signal generation units 13-1 to 13-4 based on the transfer function calculated by the transfer function estimation unit 15. Here, as a method of compensating the frequency characteristic based on the transfer function, any method can be applied. For example, digital signal processing using a FIR (Finite Impulse Response) filter, an IIR (Infinite Impulse Response) filter, or the like. A method or an analog method using an analog filter such as a phase shifter or a delay line is applied.

  The high-speed signal generation units 13-1 to 13-4 have the same internal configuration. As an example, FIG. 3 shows the internal configuration of the high-speed signal generation unit 13-1. The high-speed signal generation unit 13-1 includes two Sub-DACs 131-1 and 132-1 and an analog multiplexer 133-1. The two Sub-DACs 131-1 and 132-1 convert low-speed signals that are digital signals into analog signals. The analog multiplexer 133-1 synthesizes the analog signals converted by the two sub-DACs 131-1 and 132-1 to generate a high-speed signal. As described above, by using the two Sub-DACs 131-1 and 132-1, a request is made to each of the two Sub-DACs 131-1 and 132-1 when a high-speed signal is generated by a single DAC. The bandwidth to be used can be greatly reduced.

  The optical modulator 14 is a polarization multiplexing IQ modulator including a polarization multiplexing Mach-Zehnder vector modulator, a driver amplifier, and a laser module. In addition, in the optical modulator 14, driver amplifiers installed in the respective lanes are used for the four-lane high-speed signals (XI, XQ, YI, YQ) output from the high-speed signal generators 13-1 to 13-4. Amplification is performed, and the amplified high-speed signal is output as a modulation signal to the polarization multiplexing Mach-Zehnder vector modulator. The polarization multiplexed Mach-Zehnder vector modulator modulates the optical signal from the laser module based on the modulation signal, and outputs the modulated optical signal to the transmission line.

  The transfer function estimator 15 has the internal configuration shown in FIG. 4, and includes transfer function estimators 15-1 to 15-4 for each of four lanes of high-speed signals (XI, XQ, YI, YQ). Yes. Each of the transfer function estimation units 15-1 to 15-4 has the same internal configuration. As an example, FIG. 4 shows the internal configuration of the transfer function estimation unit 15-1. The transfer function estimation unit 15-1 includes a high-speed signal generation unit pre-signal processing unit 151-1 and two transfer function estimation circuits 152-1 and 153-1.

The high-speed signal generation unit pre-signal processing unit 151-1 has a function of processing a high-speed signal for one lane of the high-speed signal generation unit pre-signal processing unit 113 included in the transmission signal generation unit 11 described above. Yes. The high-speed signal generation unit front signal processing unit 151-1 generates low-speed signals (XI ₁ , XI ₂ ) from the high-speed signal (XI). Further, the high-speed signal generation unit front signal processing unit 151-1 outputs the low-speed signal (XI ₁ ) to the transfer function estimation circuit 152-1, and the low-speed signal (XI ₂ ) to the transfer function estimation circuit 153-1. Output.

The transfer function estimation circuit 152-1 acquires the low-speed signal (XI ₁ ) output from the transmission signal generation unit 11 as a reference signal, and the low-speed signal output from the high-speed signal generation unit pre-signal processing unit 151-1. The frequency response is calculated by synchronizing (XI ₁ ) and the low-speed reference signal (XI ₁ ) (hereinafter referred to as “reference signal”). Here, as a method of calculating the frequency response, an error between the low-speed signal (XI ₁ ) output from the high-speed signal generation unit pre-signal processing unit 151-1 and the reference signal (XI ₁ ) is minimized. There is a technique using an application filter using an LMS (Least Mean Square) algorithm or an RLS (Recursive Least Square) algorithm for optimizing a tap coefficient of a digital filter. As other methods, a ZF (Zero-Forcing) method, an MMSE (Minimum Mean Square Error) method, or the like may be applied.

Similarly, the transfer function estimation circuit 153-1 acquires the low-speed signal (XI ₂ ) output from the transmission signal generation unit 11 as a reference signal, and the low-speed output from the high-speed signal generation unit pre-signal processing unit 151-1. The frequency response is calculated by synchronizing the signal (XI ₂ ) and the reference signal (XI ₂ ). In this way, in each of the transfer function estimation units 15-2, 15-3, and 15-4, the high-speed signal (XQ), the reference signal (XQ ₁ , XQ ₂ ), the high-speed signal (YI), and the reference signal (YI) ₁ , YI ₂ ), and the transfer function is calculated by calculating the frequency response in the high-speed signal (YQ) and the reference signal (YQ ₁ , YQ ₂ ).

  As for the high-speed signal (XI, XQ, YI, YQ) reception method in the optical modulator 14 and the transfer function estimation unit 15, for example, a method using a high-speed ADC (Analog Digital Converter) or a low-speed ADC is used. Thus, there is a method for receiving a high-speed signal by changing the cycle and relative phase of the high-speed signal and repeatedly receiving data. Further, any method other than these may be applied as a high-speed signal reception method.

In the configuration of the first embodiment, the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generator 11 are used as reference signals. The transfer function estimation unit 15 calculates a transfer function from the high-speed signals (XI, XQ, YI, YQ) output from the high-speed signal generation units 13-1 to 13-4. Based on the calculated transfer function, the transfer function compensation unit 12 outputs the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂₎ output from the transmission signal generation unit 11. ) Is compensated for. As a result, the high-speed signal generation units 13-1 to 13-4 use a plurality of DACs (for example, Sub-DACs 131-1 and 132-1), and the difference in frequency characteristics between the low-speed signals and the high-speed signal generation. It is possible to compensate for the imperfection of the analog device in the units 13-1 to 13-4, and the signal quality can be improved.

In the first embodiment, the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generator 11 are used as reference signals. However, the embodiment of the present invention is not limited to the embodiment. For example, a part of the transmission data series is a known signal, the transfer function estimation unit 15 stores the known signal in advance, and calculates the transfer function when the known signal is transmitted, The transfer function can be calculated without using the low-speed signal output from the transmission signal generator 11. The transfer function estimation unit 15 determines a received symbol of the high-speed signal from the high-speed signals (XI, XQ, YI, YQ) output from the high-speed signal generation units 13-1 to 13-4, and the symbol obtained by the determination A low speed signal may be generated from the sequence, and the generated low speed signal may be used as a reference signal. As described above, when the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11 are not used as reference signals, transfer function estimation is performed. The transfer function estimation unit 15a shown in FIG. 5 may be applied instead of the unit 15, so that the transfer function estimation unit 15a and the output terminal of the transmission signal generation unit 11 are not connected.

(Second Embodiment)
FIG. 6 is a block diagram illustrating a configuration of an optical transmission system according to the second embodiment. The same components as those of the optical transmitters 10 and 10a of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. The optical transmission system 1 includes an optical transmitter 10b, an optical receiver 20, a transmission path 4 connected to the optical transmitter 10b and the optical receiver 20, and a communication connected to the optical transmitter 10b and the optical receiver 20. A line 5 is provided. The optical transmitter 10b includes a transmission signal generation unit 11, a transfer function compensation unit 12, high-speed signal generation units 13-1 to 13-4, and an optical modulator 14, and differs from the optical transmitter 10 of the first embodiment in transmission. The function estimation unit 15 is not provided.

  The transmission line 4 includes an optical fiber 4-1 and an optical amplifier 4-2, and an optical signal output from the optical transmitter 10 b is transmitted through the optical fiber 4-1 and amplified by the optical amplifier 4-2. The optical receiver 20 receives the signal. The communication line 5 is, for example, a communication channel shown in Reference Document 1 below, or a control channel such as NE-Ops (Network Element Operation System) or NW-Ops (Network Operation System) (Reference Document 1: S.S. Okamoto, M. Yoshida, K. Yonenaga, and T. Kataoka. “Adaptive Pre-equalization using Bidirectional Pilot Sequences to Estimate and Feed Back Amplitude Transfer Function and Chromatic Dispersion”, OFC2015 Th2A.29).

The communication line 5 is connected to the output terminal of the transmission signal generation unit 11 of the optical transmitter 10b and the transfer function estimation unit 15 of the optical receiver 20, and the transfer function of the transfer function estimation unit 15 and the optical transmitter 10b. It is connected to the compensation unit 12. Further, the communication line 5 uses the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11 as a transfer function of the optical receiver 20. It is used when transmitted to the estimation unit 15 and when the transfer function calculated by the transfer function estimation unit 15 is transmitted to the transfer function compensation unit 12 of the optical transmitter 10b.

  The optical receiver 20 includes a reception unit 21, a reception data demodulation unit 25, and a transfer function estimation unit 15 included in the optical transmitter 10 of the first embodiment. The receiving unit 21 includes an optical coherent receiving unit 22, ADCs (Analog to Digital Converter) 23-1 to 23-4, and a digital signal processing unit 24. The optical coherent receiving unit 22 includes a laser module therein, and converts the optical signal received from the transmission path 4 into a baseband signal by local light emission from the laser module and outputs it as an electric signal. The ADCs 23-1 to 23-4 convert analog electric signals into digital electric signals and output the digital electric signals.

  The digital signal processing unit 24 compensates for waveform degradation caused by chromatic dispersion, polarization fluctuation, and nonlinear optical effect generated in the transmission path 4. The digital signal processing unit 24 compensates for phase noise due to the frequency error of the laser on the optical transmitter 10b side and the laser on the optical receiver 20 side, and the line width of each laser, and performs high-speed signals (XI, XQ, YI, YQ) is output to the transfer function estimation unit 15 and the reception data demodulation unit 25. The reception data demodulator 25 determines the high-speed signal (XI, XQ, YI, YQ) output from the digital signal processor 24 and demodulates the binary information transmitted by the optical transmitter 10b.

The transfer function estimation unit 15 is a low-speed signal (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₁₎ generated by the transmission signal generation unit 11 of the optical transmitter 10 b received through the communication line 5. The transfer function is calculated based on the high-speed signals (XI, XQ, YI, YQ) output from the digital signal processing unit 24 using YQ ₂ ) as a reference signal. The transfer function estimation unit 15 transmits the calculated transfer function to the transfer function compensation unit 12 of the optical transmitter 10b through the communication line 5.

In the configuration of the second embodiment, the optical signal output from the optical transmitter 10b is transmitted by the transmission path 4, the optical receiver 20 receives the optical signal, and the receiving unit 21 receives the high-speed signal from the optical signal. (XI, XQ, YI, YQ) is generated. The transfer function estimation unit 15 included in the optical receiver 20 communicates the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11. The transfer function is calculated based on the high-speed signals (XI, XQ, YI, YQ) received through the line 5 and using the low-speed signal as a reference signal and output from the receiving unit 21. The transfer function estimation unit 15 transmits the calculated transfer function to the transfer function compensation unit 12 of the optical transmitter 10b through the communication line 5. Based on the received transfer function, the transfer function compensation unit 12 outputs the low speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11. Compensation is performed for. As a result, a transfer function that compensates for the frequency characteristic difference between the low-speed signals and the high-speed signals generated in the high-speed signal generation units 13-1 to 13-4 and the optical modulator 14 of the optical transmitter 10b and the imperfection of the analog device is obtained. The signal quality can be improved by performing compensation using the calculated transfer function.

(Third embodiment)
FIG. 7 is a block diagram illustrating a configuration of an optical transmission system according to the third embodiment. The same configurations as those of the optical transmitters 10 and 10a of the first embodiment and the optical transmission system 1 of the second embodiment are denoted by the same reference numerals, and different configurations will be described below. The optical transmission system 2 includes an optical transmitter 10c, an optical receiver 20c, a transmission path 4 connected to the optical transmitter 10c and the optical receiver 20c, and communication connected to the optical transmitter 10c and the optical receiver 20c. A line 5c is provided. The optical transmitter 10c includes a transmission signal generation unit 11, high-speed signal generation units 13-1 to 13-4, and an optical modulator 14, and includes a transfer function compensation unit 12 unlike the optical transmitter 10b of the second embodiment. Absent.

The optical receiver 20c includes a receiving unit 21, a high-speed signal separating unit 26, a transfer function compensating unit 12 and signal combining units 28-1 to 28- provided in the optical transmitters 10, 10a, and 10b of the first and second embodiments. 4. A reception data demodulating unit 25 and a transfer function estimating unit 15 are provided. The transfer function estimation unit 15 is a low-speed signal (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₁₎ generated by the transmission signal generation unit 11 of the optical transmitter 10 c received through the communication line 5 c. The transfer function is calculated based on the high-speed signals (XI, XQ, YI, YQ) output from the digital signal processing unit 24 using YQ ₂ ) as a reference signal. Further, the transfer function estimating unit 15 transmits the calculated transfer function to the transfer function compensating unit 12.

The high-speed signal separation unit 26 performs the same processing as the high-speed signal generation unit pre-signal processing unit 113 of the transmission signal generation unit 11 and outputs high-speed signals (XI, XQ, YI, YQ) output from the digital signal processing unit 24. Low speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) are generated.

The transfer function compensation unit 12 performs the transfer calculated by the transfer function estimation unit 15 on the generated low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ). Perform compensation processing based on the function. Each of the signal synthesis units 28-1 to 28-4 is a low-speed signal (XI ₁ , XI ₂ ), low-speed signal (XQ ₁ , XQ ₂ ), low-speed signal (YI) that has been compensated by the transfer function compensation unit 12. ₁ , YI ₂ ) and low-speed signals (YQ ₁ , YQ ₂ ), respectively, to generate high-speed signals (XI, XQ, YI, YQ). The reception data demodulation unit 25 demodulates binary information from the high-speed signals (XI, XQ, YI, YQ) generated by the signal synthesis units 28-1 to 28-4. The signal synthesizers 28-1 to 28-4 may be configured with analog parts as with the high-speed signal generators 13-1 to 13-4, or may be configured by digitally reproducing the same circuit configuration. May be.

In the configuration of the third embodiment, the optical signal output from the optical transmitter 10c is transmitted by the transmission path 4, the optical receiver 20c receives the optical signal, and the receiving unit 21 receives the high-speed signal from the optical signal. (XI, XQ, YI, YQ) is generated. The transfer function estimation unit 15 included in the optical receiver 20c communicates the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11. The transfer function is calculated based on the high-speed signals (XI, XQ, YI, YQ) received from the line 5c, using the low-speed signal as a reference signal, and output from the receiving unit 21. The transfer function estimation unit 15 outputs the calculated transfer function to the transfer function compensation unit 12. The high-speed signal separating unit 26 converts the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ ) from the high-speed signals (XI, XQ, YI, YQ) output from the digital signal processing unit 24. , YQ ₂ ). Based on the transfer function, the transfer function compensator 12 converts the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the high-speed signal separator 26. Compensation will be provided. As a result, a transfer function that compensates for the frequency characteristic difference between the low-speed signals and the high-speed signals generated in the high-speed signal generation units 13-1 to 13-4 and the optical modulator 14 of the optical transmitter 10c and the imperfection of the analog device is obtained. The signal quality can be improved by performing compensation using the calculated transfer function.

In the third embodiment, the high speed signal separation unit 26 is based on the high speed signals (XI, XQ, YI, YQ) generated and output by the digital signal processing unit 24, and the low speed signals (XI ₁ , XI). ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ). However, the configuration of the present invention is not limited to this embodiment. For example, based on the high-speed signals (XI, XQ, YI, YQ) generated by the transfer function estimation unit 15, the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ) ₂ ), and the transfer function compensation unit 12 may compensate the low-speed signal.

In the configurations of the second and third embodiments, the transfer function estimation unit 15 generates the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XI) generated by the transmission signal generation unit 11 received via the communication lines 5 and 5c. XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) are used as reference signals, but the configuration of the present invention is not limited to this embodiment. For example, a part of the transmission data series is a known signal, the transfer function estimation unit 15 stores the known signal in advance, and calculates the transfer function when the known signal is transmitted, The transfer function can be calculated without using the low-speed signal output from the transmission signal generator 11. The transfer function estimation unit 15 determines a received symbol of the high-speed signal from the high-speed signal (XI, XQ, YI, YQ) output from the digital signal processing unit 24, and generates a low-speed signal from the symbol sequence obtained by the determination. The generated low-speed signal may be used as the reference signal. As described above, when the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11 are not used as reference signals, a transmission signal is generated. The communication lines 5 and 5c that connect the output terminal of the unit 11 and the transfer function estimation unit 15 may not be provided, and the transfer function estimation unit 15a illustrated in FIG. Will be.

Further, in the second and third embodiments, the pre-dispersion compensation or pre-linear nonlinear optics for compensating for the waveform distortion of the transmission line 4 in the waveform shaping unit 112 of the transmission signal generation unit 11 of the optical transmitters 10b and 10c. When effect compensation is performed, waveform distortion generated in the transmission line 4 in the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) output from the transmission signal generation unit 11. Will be inherent. In the optical receivers 20 and 20c, the digital signal processing unit 24 compensates for the waveform distortion that occurs in the transmission line 4, so the transfer function estimation unit 15 causes the low-speed signal (XI ₁₎ in which the waveform distortion that occurs in the transmission line 4 is inherent. , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ), it is not possible to estimate only the transfer function in the optical transmitters 10 b and 10 c even if the transfer function is calculated. .

  In this case, the waveform shaping unit 112 of the transmission signal generation unit 11 outputs a signal that performs pre-dispersion compensation or pre-linear optical effect compensation, and outputs a signal that does not perform pre-dispersion compensation or pre-linear optical effect compensation. Let The high-speed signal generation unit pre-signal processing unit 113 generates a low-speed signal based on the signal subjected to pre-dispersion compensation or pre-linear optical effect compensation, and the transfer function compensation unit 12 of the optical transmitter 10b or the optical signal It outputs to the high-speed signal generation parts 13-1 to 13-4 of the transmitter 10c. On the other hand, the high-speed signal generating unit pre-signal processing unit 113 generates a low-speed signal based on a signal for which pre-dispersion compensation or pre-linear optical effect compensation is not performed, and the generated low-speed signal is, for example, the communication line 5, It transmits to the transfer function estimation part 15 through 5c. The transfer function estimator 15 uses the received low-speed signal as a reference signal when calculating the transfer function, thereby performing transmission in the optical transmitters 10b and 10c while performing pre-dispersion compensation and pre-linear optical effect compensation. Only the function can be calculated.

As another method, there is a method using a transfer function estimating unit 15d shown in FIG. 8 instead of the transfer function estimating unit 15. The transfer function estimator 15d includes transfer function estimators 15d-1 to 15d-4 corresponding to the high-speed signal lanes. The transfer function estimation units 15d-1 to 15d-4 have the same internal configuration, and FIG. 8 illustrates the internal configuration of the transfer function estimation unit 15e-1 as an example. Each of the transfer function estimation units 15d-1 to 15d-4 includes waveform shaping units 154-1 to 154-4 in front of the high-speed signal generation unit pre-signal processing units 151-1 to 151-4 included therein. ing. The waveform shaping units 154-1 to 154-4 perform the same waveform shaping as the waveform shaping unit 112 of the transmission signal generation unit 11. Based on the signals after waveform shaping by the waveform shaping units 154-1 to 154-4, the high-speed signal generation unit pre-signal processing units 151-1 to 151-4 generate low-speed signals. As a result, even if the pre-dispersion compensation or the pre-linear optical effect compensation is performed in the waveform shaping unit 112 of the transmission signal generation unit 11, the transfer function estimation unit 15d performs the same compensation process. Using the low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ ₁ , YQ ₂ ) generated by the signal generator 11, only the transfer functions in the optical transmitters 10 b and 10 c are obtained. It is possible to calculate.

  Furthermore, as another method, there is a method using a transfer function estimating unit 15e shown in FIG. 9 instead of the transfer function estimating unit 15. The transfer function estimation unit 15e includes transfer function estimation units 15e-1 to 15e-4 corresponding to high-speed signal lanes. The transfer function estimation units 15e-1 to 15e-4 have the same internal configuration, and FIG. 9 illustrates the internal configuration of the transfer function estimation unit 15e-1 as an example. The transfer function estimation unit 15e-1 includes a symbol synchronization unit 155-1, a waveform shaping unit 154-1, a high-speed signal generation unit pre-signal processing unit 151-1, and transfer function estimation circuits 152e-1 and 153e-1. . The waveform shaping unit 154-1 has the same configuration as that of the transfer function estimation unit 15d described above, and the high-speed signal generation unit pre-signal processing unit 151-1 also includes transfer function estimation units 15, 15a, and 15d. It is the same structure as what is provided.

  As a premise for applying the transfer function estimating unit 15e, one end of a line connecting the output terminal of the transmission function estimating unit 15 and the transmission signal generating unit 11 among the communication lines 5 and 5c shown in FIG. 6 and FIG. Instead of the output terminal of the unit 11, it is assumed to be connected to the output terminal of the bit mapping unit 111 of the transmission signal generation unit 11. The symbol synchronization unit 155-1 performs symbol synchronization with the high-speed signal (XI) output from the digital signal processing unit 24 using the symbol information received from the bit mapping unit 111 as a reference signal. The symbol synchronization unit 155-1 outputs the high-speed signal (XI) output from the digital signal processing unit 24 and the high-speed signal (sXI) subjected to symbol synchronization to the waveform shaping unit 154-1.

The waveform shaping unit 154-1 performs the same waveform shaping as the waveform shaping unit 112 of the transmission signal generation unit 11 on the high-speed signal (XI) and the high-speed signal (sXI). The high-speed signal generation unit pre-signal processing unit 151-1 generates four low-speed signals (XI ₁ , sXI ₁ , XI ₂ , sXI ₂ ) based on the signal after waveform shaping by the waveform shaping unit 154-1. Each of the transfer function estimation circuits 152c- ₁ and 153c- ₁ calculates a transfer function based on the low speed signals (XI ₁ , sXI ₁ ) and the low speed signals (XI ₂ , sXI ₂ ). Similarly, the transfer function estimation units 15e-2 to 15e-4 calculate the transfer function. As a result, even if pre-dispersion compensation or pre-linear optical effect compensation is performed in the waveform shaping unit 112 of the transmission signal generation unit 11, the bit mapping unit 111 before performing these compensation processes outputs it. By using the symbol information, the transfer function estimation unit 15e performs transfer functions in the optical transmitters 10b and 10c while performing pre-dispersion compensation and pre-linear optical effect compensation in the waveform shaping unit 112 of the transmission signal generation unit 11. Only can be calculated.

In the second and third embodiments, the digital signal processing unit 24 outputs the digital signal before waveform equalization to the transfer function estimating unit 15, and the waveform distortion generated in the transmission path 4 in the transfer function estimating unit 15. The transfer function may be calculated after compensation.
In the second and third embodiments, a path switching machine may be installed in the transmission path 4.

(Experimental result)
FIG. 10 shows the effect of the experiment using the optical transmission system 1 of the second embodiment. Polarization multiplexing 16QAM (DP-16QAM) was used as the modulation method, and the modulation rate was 96 GBaud. The transmission line 4 was tested as a Back-to-Back condition. In this case, in the high-speed signal generation units 13-1 to 13-4, the low-speed signal is given to the Sub-DACs 131-1 to 131-4 and 132-1 to 132-4 at a sampling rate of 96GSSample / sec. The analog multiplexers 133-1 to 133-4 combine the outputs from the Sub-DACs 131-1 to 131-4 and 132-1 to 132-4 and output them as 96 GBaud high-speed signals.

  In FIG. 10, the horizontal axis represents optical signal-to-noise power (OSNR), and the vertical axis represents the reception Q value. In FIG. 10, a solid line is a theoretical value line of 96 GBaud-DP-16QAM. Square plots are experimental results without transfer function compensation, and circle plots are experimental results when transfer function compensation is performed in the optical transmission system 1 of the second embodiment. It can be seen from the transfer function compensation that when the OSNR is 40 dB, the reception Q value is improved by about 2.5 dB, and the OSNR tolerance is improved by about 2 dB around the reception Q value of 5 dB.

  In the first, second, and third embodiments, as in the example shown in Non-Patent Document 3, two Sub-DACs 131-1 and 132-1 and one analog multiplexer 133-1 are used. Although a high-speed signal is generated, the embodiment of the present invention is not limited to the embodiment. For example, a high-speed signal may be generated by three DACs and one analog multiplexer as in the example shown in Non-Patent Document 4, or four or more DACs may be used. A configuration in which one high-speed signal is generated from two low-speed signals and a configuration in which one high-speed signal is generated from three or more low-speed signals may be mixed.

  In the configurations of the first, second, and third embodiments, the polarization multiplexed signal is used, but a single polarization signal may be used. In the configurations of the first, second, and third embodiments described above, an IQ modulated signal is used, but an intensity modulated signal may be used. In this case, the optical modulator 14 is the polarization multiplexing type Mach-Zehnder type described above. Instead of a vector modulator, an intensity modulator type or a direct modulation light source for a laser module is applied.

  In the configurations of the first, second, and third embodiments described above, QPSK and 16QAM symbol points are applied as symbol points assigned by the bit mapping unit 111, but other modulation formats are applied. May be.

  In the first, second, and third embodiments, when the waveform distortion due to the frequency characteristics is large, the transfer function estimation units 15, 15a, 15d, and 15e cannot accurately estimate due to the deterioration of the signal quality. There are cases. In that case, the calculated transfer function is fed back to the transfer function compensation unit 12 and the calculation of the transfer function is repeated, whereby the transfer function can be compensated with higher accuracy.

Further, in the optical transmitter 10 of the first embodiment, the transfer function estimation unit 15d and the transfer function estimation unit 15e shown in FIGS. 8 and 9 are applied instead of the transfer function estimation unit 15 shown in FIG. It may be. When the transfer function estimation unit 15e shown in FIG. 9 is applied to the optical transmitter 10, low-speed signals (XI ₁ , XI ₂ , XQ ₁ , XQ ₂ , YI ₁ , YI ₂ , YQ) are output from the output end of the transmission signal generation unit 11. ₁ , YQ ₂ ), but symbol information is received as a reference signal from the output end of the bit mapping unit 111 of the transmission signal generation unit 11.

  The optical transmitters 10, 10a, 10b, 10c and the optical receivers 20, 20c in the first, second, and third embodiments described above may be realized by a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program held for a certain period of time. Further, the program may be a program for realizing a part of the above-described functions, and may be a program capable of realizing the functions described above in combination with a program already recorded in a computer system. You may implement | achieve using programmable logic devices, such as FPGA (Field Programmable Gate Array).

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

DESCRIPTION OF SYMBOLS 10 ... Optical transmitter, 11 ... Transmission signal generation part, 12 ... Transfer function compensation part, 13-1 to 13-4 ... High speed signal generation part, 14 ... Optical modulator, 15 ... Transfer function estimation part, 20 ... Optical reception 21 ... receiving unit, 22 ... optical coherent receiving unit, 23-1 to 23-4 ... ADC, 24 ... digital signal processing unit, 25 ... received data demodulating unit

Claims (8)

A transmission signal generator that generates a plurality of low-speed signals from the transmission data sequence;
Dividing the plurality of low-speed signals into a combination including two or more low-speed signals, and generating a high-speed signal by digital-analog conversion of the plurality of low-speed signals to synthesize each combination; and
A transfer function estimator that calculates a transfer function based on the generated high-speed signal and a reference signal;
A transfer function compensator that compensates the plurality of low-speed signals generated by the transmission signal generator based on the transfer function and outputs the compensated signals to the high-speed signal generator;
An optical transmitter. The transfer function estimator is
The transfer function is calculated by using the plurality of low-speed signals generated by the transmission signal generation unit as the reference signal, or a part of the transmission data sequence that is determined in advance as a known signal is used as the reference signal. The low-speed signal is generated from a symbol sequence obtained by determining the received symbol of the high-speed signal, and the transfer function is calculated using the generated low-speed signal as the reference signal. The optical transmitter described. An optical transmission system comprising an optical transmitter, an optical receiver, and a transmission path connected to the optical transmitter and the optical receiver,
The optical transmitter is
A transmission signal generator that generates a plurality of low-speed signals from the transmission data sequence;
Dividing the plurality of low-speed signals into a combination including two or more low-speed signals, and generating a high-speed signal by digital-analog conversion of the plurality of low-speed signals to synthesize each combination; and
An optical modulator that transmits an optical signal obtained by modulating the high-speed signal to the transmission line, and
The optical receiver is:
A receiver that receives the optical signal from the transmission line and outputs the high-speed signal obtained from the received optical signal;
A received data demodulator that demodulates binary information included in the transmission data sequence based on the high-speed signal output by the receiver;
Have
The high-speed signal that is provided in the optical transmitter or the optical receiver and is generated by the high-speed signal generation unit, or the high-speed signal that is one of the high-speed signals that are generated in the optical receiver, and a reference A transfer function estimator that calculates a transfer function based on the signal,
A transfer function compensator provided in the optical transmitter or the optical receiver and configured to compensate the plurality of low-speed signals generated in the optical transmitter or the optical receiver based on the transfer function; ,
An optical transmission system comprising: The transfer function estimator is
The transfer function is calculated by using the plurality of low-speed signals generated by the transmission signal generation unit as the reference signal, or a part of the transmission data sequence that is previously determined as a known signal is transmitted as the reference signal. A function is calculated, or a received symbol of the high-speed signal is determined, the low-speed signal is generated from a symbol sequence obtained by the determination, and the transfer function is calculated using the generated low-speed signal as the reference signal. Item 4. The optical transmission system according to Item 3. The transfer function estimator is
When the transmission signal generation unit performs waveform shaping to compensate for waveform distortion in the transmission path, the plurality of low-speed signals not subjected to waveform shaping are used as the reference signals, or the high-speed signal The optical transmission system according to claim 3, wherein the transfer function is calculated after performing the same waveform shaping on the signal. Light transmitted from an optical transmitter that generates a high-speed signal by dividing a plurality of low-speed signals into combinations including two or more low-speed signals, and converting the plurality of low-speed signals into digital-analog signals and combining them for each combination. An optical receiver for receiving a signal,
A receiver for receiving the optical signal;
A transfer function estimator that calculates a transfer function based on the high-speed signal obtained from the optical signal received by the receiver and a reference signal;
A transfer function compensator for compensating the plurality of low-speed signals obtained from the optical signal received by the receiver based on the transfer function;
A signal synthesizer for synthesizing the plurality of low-speed signals to be compensated by the transfer function compensator to generate the high-speed signal;
A reception data demodulator that demodulates binary information included in the transmission data sequence based on the generated high-speed signal;
An optical receiver. The transfer function estimator is
The plurality of low-speed signals are received from the optical transmitter, and the transfer function is calculated using the received plurality of low-speed signals as the reference signal, or one of the transmission data sequences determined in advance as a known signal The transfer function is calculated using the reference unit as the reference signal, or the received symbol of the high-speed signal obtained from the received optical signal is determined, and the low-speed signal is generated from the symbol sequence obtained by the determination, and generated The optical receiver according to claim 6, wherein the transfer function is calculated using the low-speed signal as the reference signal. A high-speed signal separator that separates the high-speed signal obtained from the optical signal received by the receiver to generate the plurality of low-speed signals;
The transfer function compensator is
The optical receiver according to claim 6, wherein the plurality of low-speed signals generated by the high-speed signal separation unit are compensated based on the transfer function.

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